Apple has delayed the requirement for all apps in the Mac App Store to be sandboxed till March 01, 2012. The original date is November 1, apparently there are issues preventing the enforcement. This is a relief for developers with apps in the Mac App Store, most are rushing to change their apps in order to meet the sandbox requirement.
When an app is sandboxed, it is more secure to the users as the app is limited on what it can access on the Mac. This is similar to the sandboxed environment of iOS. By limiting access to system resources, app also is limited to what it can do. This is especially apparent for utilities apps which most often than not are manipulating system resources. Certain type of system access is forbidden under sandbox, thus we might see a number of existing Mac App Store apps drop out after March next year.
Sanbox requirement applies to apps on the Mac App Store. For apps that you downloaded or install via disc, it is still the same open world as before.